Electron cyclotron resonance (ECR) relates to the circular motion of free electrons orbiting about magnetic field lines while absorbing microwave energy. When the orbital frequency of the electrons in the magnetic field is equal to the frequency of the microwaves, resonance occurs and the electrons gain energy. The ECR frequency, f.sub.ECR, is: ##EQU1## where: e=electron charge
m.sub.e =mass of an electron PA1 c=speed of light PA1 B=magnetic field strength in Gauss. PA1 pi=constant, pi, 22/7.
For a given microwave frequency, f.sub.m, ECR will occur at a specific magnetic field strength which satisfies the equation: EQU f.sub.m =f.sub.ECR =(2.80.times.10.sup.6) B H.sub.z
To form a plasma with an ECR source, gas is introduced into a low pressure plasma generating chamber. A magnetic field is applied at the chamber to cause cyclatron motion of the electrons in the gas. By creating an ECR condition with the microwave beam and magnetic field, the electrons gain energy through resonant absorption and collide with the background gas causing further ionization. The result is a plasma consisting of electrons, ions, free radicals, and neutral atoms.
By continually pumping microwave energy into the chamber, the gyrating electrons are "hotter" (i.e. more energetic) than electrons generated with "hot filament" ion sources. This enables the ECR plasma source to continue to create ions and provides a high ionization efficiency which is significantly greater than that of typical arc and cathode discharge plasma ion sources.
One problem with using ECR to generate a plasma is the difficulty of achieving a uniform population distribution of the ions throughout the plasma (i.e. uniform ion density). By using only an electromagnetic coil to create the magnetic field the ion density is more concentrated at the center of the plasma, where the magnetic field is strongest, than at the edges, where the magnetic field is weaker. The effect of such a nonuniform distribution is that the plasma or ion beam extracted will also have a nonuniform ion density. A uniform ion density is desirable for etching, deposition, pre-deposition and material property modification processes.
An object of the invention is to provide a magnetic field configuration for achieving a uniform ion density.
Another object of this invention is to provide a magnetic field configuration which enhances the coupling of microwave energy and magnetic field strength to optimize the power transfer to the free electrons in a plasma.
In applications which generate an ion beam between 20 and 2000 Electron-Volts, the plasma generating chamber will be subject to a high potential.
Another object of this invention is to provide a voltage isolator between the microwave source and the plasma generating chamber so that the microwave source may operate at or near ground potential while the plasma generating chamber may operate at a high potential.
Another object of this invention is to provide a voltage isolator, pressure isolator, microwave choke, and cooling apparatus in a single window apparatus.